Very high frequency mixer

ABSTRACT

A mixer of the two-transistor type (T 1 , T 2 ) arranged in series by a source S 2  and a drain D 1 . A Gate G 1  receives a radio frequency signal RF and a gate G 2  a local oscillator signal L0. Matching networks R 1  and R 2  are provided for the respective signals RF and L0. One element of the network R 3  positioned at the output (drain D 2  of the transistor T 2 ) restores a short-circuit at the frequency f L0  of the local oscillator. The instability of the input G 2  of the transistor T 2  is compensated for by an inductor element L inserted between the junction point of the source S 2  and the drain D 1  and the common-mode pole.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a very high frequency mixer comprising a first and a second field effect transistor, one of the transistors being intended to receive at its gate a local oscillator signal and the other transistor a radio-frequency input signal, the first transistor having a source connected to a first supply voltage pole, and a drain connected at a junction point to a source of the second transistor, and the drain of the second transistor being connected to a second supply voltage pole via a resistor.

2. Description of Related Art

Such a mixer is described in the article published by Peter HARROP et al. in the publication "Proceedings of the Sixth European Microwave Conference", Rome, 14-17 September 1976, pp. 8-13.

This mixer is designed to operate within the range of approximately 7-9 GHz.

The applicants have established that the higher the cut-off frequency f_(T) of the transistor became the more unstable became the input at the gate of the second transistor.

The applicants have been able to establish that this instability was due to the drain-source capacitance of the two transistors, on the one hand, and, on the other hand, to the effect of the cut-off frequency f_(T) of the transistors, their gate-drain capacitance having no determining effect on this instability.

SUMMARY OF THE INVENTION

The subject invention proposes to realize a compensation which permits to control this instability.

With this object, the mixer according to the invention is characterized in tht the drain of the second transistor is charged by a circuit element realizing a short-circuit at the frequency of the local oscillator signal, and in that it comprises an inductor element inserted between the junction point and a common-mode pole, the value of the inductor element being selected such as to compensate for the drain-source capacitances of the transistors and thus to eliminate the instability of the input at the gate of the second transistor.

In an advantageous embodiment, the first transistor is intended to receive at its gate the radio-frequency signal, and the second transistor is intended to receive at its gate the local oscillator signal.

The gates of the first and/or second transistor(s) can include at the input a matching circuit for the radio-frequency signal and/or the local oscillator signal.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be better understood by means of the following description given by wy of non-limiting example with reference to the drawings, in which:

FIG. 1 shows a mixer in accordance with the prior art;

FIG. 2 shows a mixer in accordance with the present invention;

FIG. 3 shows an equivalent diagram representing the opration of the circuit shown in FIG. 1, having in a dotted line the inductor L arranged according to the present invention (FIG. 2);

FIG. 4 shows an embodiment of the mixer as shown in FIG. 2; and

FIGS. 5 and 6 shw a comparison between a circuit according to the invention and a circuit according to the prior art, in the neighborhood of 15 GHz and having an intermediate frequency of 200 Mhz, as far as the gain curves are concerned as a function of the power of the local oscillator and the frequency, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT

According to FIG. 1, a mixer described in the above-mentioned article comprises a field effect transistor T₁ on a substrate AsGa, in this case a transistor of the MESFET type, having a source S₁ connected to a common-mode pole, a gate G₁ receiving a local oscillator signal LO, and a drain D₁ connected to the source S₂ of a field effect transistor T₂ on an AsGa substrate, in this case also of the MESFET type. The transistor T₂ receives at its gate G₂ a radio-frequency signal RF and produces on its output, constituted by its drain D₂, an intermediate-frequency signal IF.

Between the gates G₁ and G₂ and the common-mode pole are arranged two feeding networks comprising an inductor and a capacitor connected in series, that is to say (L₁, C₁) for the transistor T₁, and (L₂, C₂) for the transistor T₂, the junction point for the two elements of each network receiving a gate bias voltage Vg.

Finally, between the drain D₂ of the transistor T₂ and the common-mode pole, is arranged a feeding network comprising an inductor L₃ and a capacitor C₃, connected in series. te junction point of the inductor L₃ and the capacitor C₃ is biassed by a drain bias voltage Vd.

Such a circuit is designed having transistors comprising a gate of approximately 0.8 micron in length.

If one wishes to realize a mixer operating at a higher frequency, and therefore uses transistors whose intrinsic cut-off frequency is much higher than in the preceding case, the applicants have established that an instability appeared at the gate G₂. This instability can be compensated by mismatching the input of the gate G₂, but at the cost of a considerable increase, the power of the signal injected into this gate; a power of approximately 20 mV has been found to be necessary for maintaining a correct gain factor between the intermediate-frequency output signal FI and the radio-frequency input signal RF.

According to FIG. 2, a transistor T₁ receives the radio-frequency signal RF via a matching network R₁, and the transistor T₂ receives the local oscillator signal LO via a matching network R₂, these networks including the power supplies. As shown in the article by Christos TSIRONIS et al., published in IEEE Transactions on microwave Theory and Techniques, Vol. MTT 32, No.3, March 1984, pp. 248-255, this drive mode is more advantageous because non-linearities occur essentially in the field effect transistor T₂ (trnsconductance gm₂ and channel resistance Rd₂) and the field effect transistor T₁ only amplifies the radio-frequency signal.

The high frequencies (LO, RF) are short-circuited using an elementary network R₃ which can be, for example, a λ/4 open microstripline (λ designating the wavelength corresponding with the local oscillator frequency) or also a capacitor having a low value (approximately 1 pF) of the interdigitated type being in series resonance at the local oscillator frequency (cf.the article by Christios TSIRONIS et al., p. 249 II).

According to the invention, the input at gate G₂ of transistor T₂ is stabilized by arranging an inductor element L, having a conveniently chosen value, between the junction point of the drain D₁ of the transistor T₁ and the source S₂ of the transistor T₂, and the common-mode pole.

The structure of the circuit of FIG. 2 can be given in a first approximately by the circuit of FIG. 3. The transistors T₁ (T₂) are determined by the capacitors Cgs₁ (Cgs₂), the voltage-controlled current sources I₁ (I₂) and the output impedances y₁ (y₂). The drain of the second transistor T₂ is charged via an open circuit line R₃ which restores a short-circuit at the local oscillator frequency LO and by a resistor R connected to the common-mode pole. The radio-frequency signal RF injected into the capacitor Cgs₁ is amplified by the first transistor T₁ and injected into the source of the second transistor T₂ where the non-linearities, due to the strong signal from the local oscillator LO injected into the capacitor Cgs₂ of the second transistor T₂, generate the intermediate-frequency signal F_(I) which is taken-off at the drain of the second transistor T₂.

We have:

    y.sub.1 =gd.sub.1 +jωCds.sub.1

    y.sub.2 =gd.sub.2 +jωCds.sub.2

with

gd₁, gd₂ =drain-source conductance of the respective transistors T₁ and T₂.

CdS₁, CdS₂ : drain-source capacitance of the respective transistors T₁ and T₂.

At the local oscillator frequency LO, the input impedance Zi of the gate G₂ of the transistor T₂ can then be described as (disregarding the inductor element L): ##EQU1## with gm₂ =transconductance of the transistor T₂

Ggs₂ =gate source capacitance of the transistor T₂

The instability is due to the fact that the real term again has a negative value when transistors are used having a high cut-off frequency f_(T).

In fact, the cut-off frequency fT₂ of the transistor T₂, which is defined as the frequency at wich its gain is 0 dB, has for its value: ##EQU2##

The instability occurs when:

    A=gd.sub.1 +gd.sub.2 -gm.sub.2 (Cds.sub.1 +Cds.sub.2)-Cgs.sub.2 <0

    or

    A=gd.sub.1 +gd.sub.2 -2πfT.sub.2 (Cds.sub.1 +Cds.sub.2)<0.

The higher the cut-off frequency fT₂ of transistor T₂ is, the more important becomes the instability.

A solution for eliminating this instability consists in mismatching the input G₂ of the transistor T₂ and introducing a resistor which permits to render the resistive term of the input impedance Zi positive. This solution has the disadvantage of considerably increasing the required power of the local oscillator signal LO.

According to the invention, the inductor element L is arranged between the drain D₁ interconnected with the source S₂, and the common-mode pole.

The expression of the input impedance Zi then becomes (always at the local oscillator frequency LO: ##EQU3##

The instability will be compensated when: ##EQU4## that is to say: ##EQU5## with Cds=Cds₁ +Cds₂

gd=gd₁ +gd₂

ω₀ =2πFo

Fo=the highest local oscillator frequency LO.

When the instability is compensated in this manner, it is possible to match the input with the gate G₂ and thus control the mixer with minimum power.

By using the inductor element L, other advantages can also be obtained.

In the first place, the effect of the presence of this inductor is the fact that the center of the arrangement (D₁, S₂) is brought back to the direct current common-mode pole, which permits biasing the transistors T₁ and T₂ independently.

Then, the inductor L short-circuits the intermediate frequency IF noise generated by the first transistor T₁ of the mixer, which is particularly interesting in the case of very high frequency transistors, for example of the MESFET type.

The low-frequency noise generated by the transistor T₁ is amplified by the transistor T₂.

The value of the voltage V at the drain D₁ of the first transistor (source S₂ of transistor T₂) is tiven by the following formula: ##EQU6## with I₀ =I₁ =noise current generated by the first transistor T₁.

Worded differently, the inductor L permits realizing a bandpass filter whose central frequency is situated within the useful receiver band, so as to filter the noise situated in the intermediate-frequency band and generated by the first transistor T₁.

Finally, the inductor L can be chosen for matching the output at the drain D₁ of the transistor T₁ with the input at the source of transistor T₂.

The matching condition can be written as: ##EQU7## ω_(RF) =2πF_(RF), F_(RF) denoting the frequency received, and F_(RF) ≠F_(LO).

In this case, A'=gd>0.

According to FIG. 4, the mixer comprises two stages, a first stage having the actual mixing function and a second stage forming a buffer.

The source S₁ of transistor T₁ is connected to the common-mode pole via a by-pass capacitor C₁₀, the source S₁ being fed by a negative voltage source V_(S1), and the drain D₂ of the transistor T₂ to the common-mode pole via two elements in a series arrangement, that is to say a resistor R₁ and a by-pass capacitor C₃, the junction point of these two elements being fed by a bias voltage drain source V_(D2).

The inputs of the radio-frequency signal RF and the local oscillator signal LO are realized via the very high frequency connecting capacitors C'₁ and C'₂, respectively. The input matching network R₁ at to the gate G₁ comprises an inductor element L'₁, arranged in series with the capacitor C'₁ and connected to the gate G₁, and, between the junction point of the capacitor C'₁ and the inductor element L'₁, on the one hand, and the common-mode pole, on the other hand, it comprises an inductor element L₁ and a very high frequency by-pass capacitor C₁. The junction point of the inductor element L₁ and the capacitor C₁ receives a gate bias voltage Vg₁. The input matching network R₂ at the gate G₂ comprises an inductor element L'₂ arranged in series with a capacitor C'₂ and connected to the gate G₂, and, between the junction point of the capacitor C'₂ and the inductor element L'₂, on the one hand, and the common-mode pole, on the other hand, it comprises an inductor element L₂ and a very high frequency by-pass capacitor C₂. The junction point of the inductor element L₂ and the capacitor C₂ receives a gate bias voltage Vg₂.

The network R₃ is represented by a transmission line SC, λLO/4 in length (λLO denoting the wavelength of the local oscillator frequency in the substrate).

An intermediate frequency IF connecting capacitor C₄ is inserted between the drain D₂ of transistor T₂ and the gate G₃ of a field effect transistor T₃. The gate G₃ is biassed by a voltage VD₁ applied to the common terminal of an intermediate-frequency by-pass capacitor C₅ and a resistor R₂. The remaining terminal of the capacitor C₅ is connected to the common-mode pole and the remaining terminal of resistor R₂ to the gate G₃. The drain D₃ of the transistor T₃ receives a drain bias voltage V_(D3) and is connected to the common-mode pole via an intermediate-frequency by-pass capacitor C₆. The source S₃ of the transistor T₃ is connected to the drain D₄ of a transistor T₄ whose source S₄ is connected to the common-mode pole and whose gate G₄ is connected to the common-mode pole.

The intermediate-frequency signal S_(IF) is obtained at the junction point of S₃ and D₄ after passing through an intermediate-frequency connecting capacitor C₇.

The FIGS. 5 and 6 show the results which can be obtained with the mixer as shown in FIG. 4 compared to the arrangements not comprising the inductor element L₁ and for which one is forced to purposely mismatch the input at the gate G₂ by leaving out the inductor element L'₂ and by substituting a resistor for the inductor element L₂.

FIG. 6 shows the gain expressed in dB as a function of the power P of the local oscillator signal, for an intermediate-frequency value f_(IF) of 200 MHz and a local oscillator frequency value f_(LO) of 15 GHz. The curve I corresponds with the prior art and the curve II with the circuit of FIG. 4. The required mismatching of the input at the gate G₂ necessitates a much higher power of the oscillator signal than the prior art circuits.

FIG. 6 shows the gain expressed in dB as a function of the frequency, a curve III corresponding to a power P_(LO) of the local oscillator signal LO of 5.5 mW (7.5 dBm) for a prior art circuit, and the curve IV to a power P_(LO) of 1.6 mW (2 dBm) for the circuit of FIG. 4, with in the two cases an intermediate-frequency f_(IF) of 200 MHz. The curve IV, although it corresponds to a power injected by the local oscillator which is more than two times smaller, corresponds to gains which are higher than those of the curve III.

The invention is not restricted to the embodiments described and shown. For example, the inductor elements can be realized in different manners; inductor or transmission line known by the English names of microstrips or coplanar lines.

A mixer according to the invention is integrable on substrate Al₂ O₃ and monolithically integrable on the GaAs substrate. 

What is claimed is:
 1. A very high frequency mixer comprising a first and a second field effect transistor, one of the transistors being intended to receive at its gate a local oscillator signal and the other transistor a radio-frequency input signal, the first transistor having a source connected to a first supply voltage pole, and a drain connected at a junction point to a source of the second transistor, and a drain of the second transistor being connected to a second supply voltage pole via a resistor, characterized in that the drain of the second transistor is charged by a circuit element realizing a short-circuit at the frequency of the local oscillator signal, and in that the mixer further comprises an inductor element inserted between the junction point and a common-mode pole, the inductor element being selected such as to compensate for drain-source capacitances of the two transistors and thus to stabilize the input at the gate of a second transistor.
 2. A mixer as claimed in claim 1, wherein the first transistor is intended to receive at its gate the radio-frequency signal and the second transistor is intended to receive at its gate the local oscillator signal.
 3. A mixer as claimed in claim 2, the gate of the first transistor comprises a circuit for matching the signal which it is intended to receive.
 4. A mixer as claimed in claim 3, wherein the gate of the second transistor (T₂) comprises a matching circuit for the signal which it is intended to receive.
 5. A mixer as claimed in one of the preceding claims 1-4 wherein the inductor element has a value L such that: ##EQU8## with: fT₂ being the cut-off frequency at 0 dB of the second transistor,Cgs₂ being the gate source capacitor of the second transistor Cds₁ and Cds₂ being drain-source capacitors of the first and second transistors, respectively, gd₁ and gd₂ being conductances of the first and second transistors, respectively, and Cds=CDs₁ +Cds₂, gd=gd₁ +gd₂ ω.sub. = π F₀ F_(o) =the highest local oscillator frequency.
 6. A mixer as claimed in claim 5, wherein the inductor element L is chosen such as to realize a bandpass filter whose central frequency is situated substantially within the useful receiver band, so as to filter out noise situated in the intermediate-frequency band and generated by the first transistor T₁.
 7. A mixer as claimed in claim 5, wherein the inductor element has a value L in the neighborhood of LO with: ##EQU9## where w_(RF) =2πF_(RF), F_(RF) denoting the frequency of the input signal and F_(RF) =F_(LO). 